1. Field of the Invention
This invention relates to an improved electrode for a plasma arc torch and, more particularly, to method and apparatus for cooling the electrode element to extend its operational life.
2. Description of the Prior Art
In a plasma arc torch, a plasma arc is developed by passing an electric arc through a constricting passageway of a nozzle located between an electrode and the workpiece. A gas is directed through the nozzle and the electric arc heats the gas to an ionization temperature. A plasma arc is emitted as a jet from the nozzle to the workpiece. The plasma arc is formed at temperatures that range from 6,000.degree. F. and higher. A plasma arc at this temperature is useful for cutting and welding ferrous and non-ferrous metals.
The electrode includes an electrically conductive casing or body portion, preferably of copper, having a generally tubular configuration with an internal bore. At the extreme lower end of the electrode tubular body portion is retained an electrode element fabricated of a high electron emitting refractory material, such as tungsten, hafnium, zirconium, or alloys thereof. The electrode element is frictionally retained to extend to the end of the body portion of the electrode into the arc chamber oppositely of the nozzle. The electrode element serves as the cathode and the nozzle serves as the anode in the electric circuit. With gas flowing into the arc chamber, the initiation of an arc between the electrode element and the nozzle heats the gas to its ionization temperature.
The extreme heat generated by the plasma arc rapidly wears the electrode element and the surrounding electrode body portion requiring that the entire electrode assembly be replaced after relatively few pierces per inch of cut into the metal. A number of measures have been taken to extend the life of an electrode by controlling the current to the electrode, controlling the orifice size of the nozzle, and changing the composition of the gas and the flow rate of the gas. Also, it is well known to circulate coolant in heat transfer relationship with the electrode element.
U.S. Pat. No. 3,450,926 discloses an electrode assembly in which the copper casing surrounding the tungsten electrode element is threaded to increase the surface area of the casing in contact with the gas stream flowing to the nozzle. The threaded surface area promotes the transfer of heat from the tungsten electrode element through the copper casing to the gas. In this manner, the gas is preheated and the electrode element is cooled to promote the life of the electrode.
U.S. Pat. No. 3,450,926 also discloses a tube that extends concentrically within the tubular electrode body portion. Water circulates through the interior of the electrode body portion into contact with the electrode element and out the tube. Circulating coolant further serves to transfer heat away from the electrode element.
The use of a coolant, liquid or gas, circulating through the interior of an electrode body portion to transfer heat from the electrode element is disclosed in U.S. Pat. Nos. 2,906,858; 3,534,388; 3,619,549; 3,825,718; and 4,769,524.
One factor to consider in utilizing a liquid coolant for an electrode assembly is preventing the liquid from reaching its boiling point. For example, as disclosed in U.S. Pat. Nos. 3,641,308 and 4,777,343, the coolant flowing through the inner tube within the electrode assembly flows in direct contact with the electrode element so that the heat is directly transferred from the element to the coolant. There is no dissipation of heat through the body portion of the electrode holder before it contacts the coolant.
It has been proposed to provide a heat transfer path between the coolant circulating through the electrode assembly and the electrode element. U.S. Pat. No. 4,311,897 and Canadian Patent No. 1,125,385 disclose electrode assemblies having inner cooling tubes in which the electrode element is separated by a heat transfer path in the body of the electrode from the liquid circulating through the tubular electrode body. With this arrangement, the bottom wall of the interior chamber of the electrode body is spaced by a mass of metal from the electrode element. By controlling the distance the electrode element is spaced from the bottom wall the rate of heat transfer from the insert to the coolant is controlled.
U.S. Pat. No. 5,208,448 and U.K. Patent No. 1,520,000 disclose another arrangement for the flow of coolant within the electrode assembly by extending a post upwardly from the bottom wall of the assembly. The electrode element extends into the post above the bottom wall. The coolant circulates around the post and increases the heat transfer efficiency from the electrode element through the post to the coolant.
While it has been proposed to extend the life of an electrode assembly in a plasma arc torch by circulating coolant through the interior of the electrode body in direct contact with the electrode element or in contact with heat transfer path to the insert, the known devices have limited heat transfer efficiency between the coolant and the electrode element. Therefore, there is need for an electrode assembly that promotes efficient transfer of heat from the electrode element to the coolant.